Revert "Allow user to send args to cargo fmt, not just to rustfmt"

This actually broke the hook. The -- needs to be present for pre-commit
to be able to pass the file names to run the hook on.

This reverts commit 4bb2cb56a1.

.pre-commit-hooks.yaml

@@ -1,7 +1,7 @@
 -   id: fmt
     name: fmt
     description: Format files with cargo fmt.
-    entry: cargo fmt
+    entry: cargo fmt --
     language: system
     types: [rust]
     args: []

hooks.yaml

@@ -1,7 +1,7 @@
 -   id: fmt
     name: fmt
     description: Format files with cargo fmt.
-    entry: cargo fmt
+    entry: cargo fmt --
     language: system
     files: \.rs$
     args: []